It's unfortunate that they restrict the number of cores according to license tier. I was excited about R integration, which is where I do a lot of parallel computations, until I realized this.
I doubt that limit affects your use case. The limit is on the number of mathematica mathkernel processes, not on the number of processes the kernel calls out to. Though, it likely takes some fiddling to set up numerous R processes per mathematica mathkernel.
M won't run if your machine has more than a certain number of cores, or you'll only be able to fill a subset of them with M kernels? I suspect the number of cores available to the slave R process is unrestricted because it runs separately.
